In recent years, a technique for manufacturing a thin film transistor (hereinafter referred to as a TFT) over a substrate has made a great progress, and application development to an active matrix display device has been advanced. In particular, a TFT formed using a poly-crystalline semiconductor film is superior in field-effect mobility (also referred to as mobility simply) to a TFT formed using a conventional amorphous semiconductor film, and therefore high-speed operation becomes possible when the TFT is formed using the poly-crystalline semiconductor film. For this reason, it has been tried to control a pixel by a driver circuit formed over the same substrate as the pixel, which was controlled conventionally by a driver circuit provided outside the substrate.
A substrate used in a semiconductor device is expected to be a glass substrate rather than a quartz substrate in terms of cost. However, the glass substrate is inferior in heat resistance and easy to change in shape due to the heat. Therefore, when the TFT using the poly-crystalline semiconductor film is formed over the glass substrate, a laser annealing method is employed to crystallize a semiconductor film formed over the glass substrate in order to prevent the glass substrate from changing in shape due to the heat.
Compared with another annealing method which uses radiation heat or conduction heat, the laser annealing has advantages that the processing time can be shortened drastically and that a semiconductor substrate or a semiconductor film over a substrate can be heated selectively and locally so that the substrate is hardly damaged thermally. It is noted that the laser annealing herein described indicates a technique to anneal a damaged layer or an amorphous layer in a semiconductor substrate or a semiconductor film and a technique to crystallize an amorphous semiconductor film formed over a substrate. Moreover, the laser annealing includes a technique applied to planarize or modify the surface of the semiconductor substrate or the semiconductor film.
As the laser oscillators used in the laser annealing, there are a pulsed laser oscillator and a continuous wave laser oscillator according to the oscillation method. In recent years, it has been known that the crystal grain formed in the semiconductor film becomes larger when using the continuous wave laser oscillator such as an Ar laser or a YVO4 laser than when using the pulsed laser oscillator such as an excimer laser in crystallizing the semiconductor film. When the crystal grain in the semiconductor film becomes larger, the number of crystal grain boundaries in the channel region of the TFT formed using the semiconductor film decreases, and the carrier mobility becomes higher so that the more sophisticated device can be developed. For this reason, the continuous wave laser oscillator is attracting attention.
A laser beam having a wavelength in the visible or ultraviolet light region is often employed in the laser annealing to the semiconductor film because such a laser beam is sufficiently absorbed in the semiconductor film. However, a solid-state laser medium generally used in the CW (continuous wave) laser emits a wavelength in the range of red to near-infrared light regions, which is not sufficiently absorbed in the semiconductor film. Therefore, the laser beam emitted from the CW laser is converted into a harmonic having a wavelength in the visible light region or shorter by a non-linear optical, element. Usually, the fundamental wave in the near-infrared light region in which high power is easily obtained is often converted into a green laser beam of the second harmonic because this method has the highest conversion efficiency.
For example, when the semiconductor film is crystallized in such a way that the CW laser beam with a power of 10 W having a wavelength of 532 nm is shaped into a linear beam having a length of approximately 300 μm in a long-side direction and approximately 10 μm in a short-side direction and that the linear beam spot is scanned in the short-side direction of the linear beam, the width of a region where the large crystal grain is obtained by one scanning of the linear beam spot is approximately 200 μm. The region where the large crystal grain is obtained is hereinafter referred to as a large crystal grain region. For this reason, to crystallize the whole surface of a semiconductor film formed over a comparatively large substrate by the CW laser beam, it is necessary to perform laser annealing by moving the position of the linear beam spot in its long-side direction by the width of the large crystal grain region obtained by one scanning of the linear beam spot.
On the other hand, at the same time as the formation of the large crystal grain region, a crystal region, which is not the large crystal grain region, (hereinafter referred to as an inferior crystallinity region) is formed at opposite ends of the linear beam in the long-side direction where the energy is attenuated. The inferior crystallinity region has a concavoconvex surface and is not suitable for manufacturing TFT thereover. When a TFT is formed using the inferior crystallinity region, the variation of electrical characteristic and operation error occur. Consequently, in order to manufacture a TFT having high reliability, it is necessary to determine correctly the region where the TFT is manufactured so that the TFT is not manufactured in the inferior crystallinity region. However, even after taking such a measure, the inferior crystallinity region still expands as the linear beam is made longer in the long-side direction.
As a result, the region where the TFT can be formed relative to the whole substrate decreases, and it is difficult to manufacture a semiconductor device with high degree of integration. The above problem is considered to result from Gaussian intensity distribution of the laser beam to be used. The Gaussian distribution has highest intensity in the center of the beam spot and lower intensity toward the opposite ends of the beam spot. Therefore, when the linear beam is made longer in the long-side direction, its end portions are extended accordingly, which results in the expansion of the inferior crystallinity region.
To reduce the inferior crystallinity region, the Gaussian intensity distribution may be changed into a top-flat shape. A laser manufacturer has introduced in its catalog a method for forming the top-flat shape with the use of a diffractive optical element or an optical waveguide. By forming the top-flat shape, the intensity distribution can have steep edges, and thus the inferior crystallinity region formed after the laser annealing can be reduced drastically. Moreover, with the top-flat intensity distribution, the inferior crystallinity region does not increase even when the linear beam is made longer in the long-side direction.
As mentioned above, the top-flat intensity distribution has the advantages. However, among the introduced methods for forming the top-flat shape, the diffractive optical element has some disadvantages of its high cost and technical difficulty because it requires fine processing on the order of nanometer to obtain the good characteristic. Moreover, the optical waveguide has a disadvantage that interference fringes appear on the irradiation surface due to the high and low intensity of the laser beam because the laser beam with a wavelength of 532 nm has high coherency.
Thus, the method for forming the top-flat shape which can avoid the problem due to the Gaussian distribution has several disadvantages. Consequently, the present inventors has developed another method for avoiding the problem due to the Gaussian intensity distribution.